Blender 2.8 "Shadeless" material setup not shadeless
The material is shadeless. However the rendered output isn't 1:1 the RGB value of the emission shader due to color management. If you were to change the View Transform in the Color Management panel to Raw you'd have perfectly white pixels. This is however not recommended for renders where the result should resemble anything like a (virtual) photograph. See the explanation of Troy Sobotka on color management in Blender.